Create pairings only once https://play.golang.org/p/UFOP4OTm7r

paironce.go

package main

import (
	"fmt"
	"os"
)

func main() {
	a := []int{1, 2, 3, 4, 5}
	for len(a) > 0 {
		for i := 1; i < len(a); i++ {
			fmt.Fprintln(os.Stderr, a[0], a[i])
		}
		a = append(a[:0], a[1:]...)
	}
}